1. Field of the Invention
One aspect of the present invention generally relates to methods for producing prototype tools and, more specifically, to methods for spray forming metal deposits.
2. Background Art
Spray forming has become an accepted technique for producing prototype tooling, i.e., dies and molds, in substantially less time than needed to make prototype tooling conventionally. The typical spray forming technique includes the following steps: (1) casting a ceramic substrate containing a spraying pattern about a mold containing a master pattern of the tool to be produced; (2) preheating the ceramic substrate; (3) spraying metal particles onto the substrate containing the spraying pattern; (4) allowing the sprayed metal particles to form a metal deposit having the general shape of the master pattern; and (5) separating the metal deposit from the ceramic substrate.
During the early stages of the spraying step, a thin shell of sprayed metal is deposited onto the spraying pattern. Significant thermal gradients can form across the thin shell. The formation of thermal gradients can be largely attributed to the following factors: (1) the difference between the temperature of the spraying pattern and the deposited metal particles, and (2) the superior insulating properties of the ceramic substrate. Typically, the ceramic substrate is preheated to about 250xc2x0 C. During the initial stages of the spraying step, the temperature of the thin shell and spraying pattern can typically reach temperatures of about 350xc2x0 C. The significant amount of heat associated with these temperatures is not dissipated through the ceramic substrate due to its superior insulating properties. Most of the heat generated by the sprayed metal particles is contained in the metal deposit.
The significant thermal gradients can cause the thin shell to separate from the spraying pattern, causing surface imperfections, i.e., wrinkles and/or cracks, in the metal deposit. After separating the metal deposit from the ceramic substrate, the surface imperfections have to be corrected with additional processing steps so that the metal deposit is suitable for prototype tooling. For example, a welding material can be (1) welded onto the surface imperfection, and (2) ground to reflect the general shape characteristics of the master pattern. As another example the surface imperfection can be smoothed and filled with a filler material, i.e., metal filled epoxy. In some cases, the surface imperfections are so extreme that the metal deposit is unsuitable for prototype tooling and must be scrapped.
To avoid the cost and time associated with the additional processing steps and scrapping, a method for spray forming metal deposits that minimizes the formation of surface imperfections is needed.
At least one aspect of the present invention is related to methods for spray forming metal deposits.
One aspect of the present invention includes providing a ceramic substrate having a spraying pattern for receiving sprayed metal particles, spraying metal particles onto the spray pattern to form a metal deposit on the spraying pattern for at least a first period, and controlling the spraying step during the first spray period so that temperature of the deposited metal particles increase at an average rate of less than or equal to about 15xc2x0 C. per minute. The first spray period lasts until the temperature of the deposited metal particles is at or about a steady state temperature. In accordance with the preferred embodiment, the controlling of spraying step comprises using at least one thermal spray gun to spray metal particles. The current of at least one thermal spray gun can increase from a first ampere value towards the second ampere value during the first spray period. The first ampere value can be 150 amperes and the second ampere value can be 220 amperes. The wire feed rate of at least one thermal spray gun can be increased from a first value to a second value during the first spray period. The first value can be about 15 pounds per hour and the second value can be about 22 pounds per hour. The steady state temperature can be about 330xc2x0 C. to about 370xc2x0 C. Preferably, the metal particles are comprised of carbon steel.
These and other aspects, objects, features and advantages of the present invention will be more clearly understood and appreciated from a review of the following detailed description of the preferred embodiments and appended claims, and by reference to the accompanying drawings.